The present invention relates to a PCM (pulse code modulation) signal interface apparatus, particularly, to a PCM signal interface apparatus having capability of synchronizing a PCM reception signal to the phases of the clock and frame in a signal processing terminal equipment and extracting and separating signaling information from the PCM reception signal.
In order to realize a signal processing terminal equipment such as a TDM/FDM converter (transmultiplexer) which interconnects time division multiplexed (TDM) PCM signals and single side-band (SSB) frequency division multiplexed (FMD) signals directly by digital signal processing, an interface apparatus for the PCM reception signals is required. The interface apparatus needs to have the following functions:
(1) Even if the clock frequency of a PCM terminal equipment which transmits a PCM signal is in synchronization with the clock frequency of a signal processing terminal equipment which receives the PCM signal, the bit phase between the clock signal of the signal processing terminal equipment and the PCM reception signal varies at random due to the jitters created during transmission. Moreover, transmission lines can vary their delay characteristics due to temperature variation. Therefore, it is required to absorb the phase difference caused by the jitters and delay variations.
(2) Even if the clock signals of both terminals are in synchronism, their timing counters are not always initialized identically, resulting in an inconsistent phase relationship between frames of a PCM reception signal and the reference time frame of a signal processing terminal. Therefore, frame alignment between them must be matched before signal processing.
(3) In a signal processing terminal, the number of data samples within one frame is sometimes required to be larger than the number of channels or words in the PCM reception signal for the sake of convenience of internal signal processing or for other reasons, and data samples need to be converted into linear codes with a large number of bits, resulting in a requirement for changing the clock rate and the frame format.
(4) It is often desired to change the channel alignment order within a frame.
(5) A PCM signal contains signaling information in addition to voice information. The signaling information is transmitted by stealing the least significant bit (LSB) of the voice sample transmitting code words every n-th frame where n is a predetermined positive integer, with the LSB being eliminated once a few frames. Therefore, PCM signal contains multi-frame synchronization information that indicates a frame carrying signaling information. Using the multi-frame synchronization information, the signaling information must be extracted, and the extracted low-speed signaling samples must be interpolated so as to provide the same frame format as that of voice samples for enabling the process in the signal processing terminal.
In order to realize digital networks, various frame aligner systems which satisfy the above-mentioned requirements (1) and (2) have been proposed. However, interface apparatus which satisfy all of the above-mentioned requirements (1)-(5) have not been developed yet.